T-Mobile Unity HB4A3 Replacement Battery 3.7V 800mAh

T-Mobile Unity / Groove / Panama — 3.7V Li-ion Replacement Battery (HB4A3)

This is a 3.7V, 800mAh Li-ion cell that replaces part number HB4A3 in the T-Mobile Unity, Groove, and Panama smartphones. All three models share the same battery footprint and connector, making this a single-part fix across the range. Capacity is rated at 2.96Wh, matching the original cell specification.

• Unity, Groove, and Panama compatibility: These three T-Mobile handsets use the same 53.00 × 34.00 × 4.80mm cell with an identical connector pinout and BMS handshake protocol — one part number covers all three without modification.
• Bench tested on actual hardware: We cycled this cell through charge and load on a Unity unit. The BMS accepted charge without fault flags, and voltage held stable under screen-on and modem-active load conditions through multiple cycles.
• Fuel gauge recalibration on first cycle: After installation, run one full discharge to automatic shutoff, then charge uninterrupted to 100% before normal use. This resets the coulomb counter against the new cell's discharge curve so percentage readings stay accurate from day one.

Why the Unity reports the wrong battery percentage after a cell swap

The fuel gauge IC on these T-Mobile handsets builds its percentage model from historical charge and discharge data tied to the old cell. When a new cell goes in, that learned curve no longer matches the actual voltage-to-capacity relationship of the replacement. The OS reads a voltage that maps to the wrong percentage on the old curve, so the number on screen drifts. One full discharge-charge cycle forces the gauge IC to rebuild its model against the new cell, correcting the readout.

Sudden shutdown at 20–30% on the replacement cell

This happens when the cell voltage drops sharply under high-current draw — modem activity or screen brightness spikes — before the gauge IC has recalibrated. The phone sees a voltage cliff and interprets it as critical low battery, triggering an emergency shutdown even though the gauge still shows charge remaining. It is not a defective cell; it is an uncalibrated coulomb counter. Complete the first full discharge cycle and charge to 100% without interruption — after that, the IC tracks the real cutoff voltage and the shutdowns stop.